The present invention relates to a signal processing circuit for a video camera, or more in particular to a clamp circuit for adjusting the balance of the black level of the video signal of the color video camera.
In a color video camera, waveform-shaping processes for the video signal are generally required including the addition of a pedestal component, the gamma correction and the white level clipping. For performing these processes, it is necessary to determine a reference value, and therefore a circuit for performing these processes is required to include a clamp circuit.
Conventionally, the clamping process in such a processing circuit is accomplished in the manner mentioned below. First, a light mask called the optical black mask is provided at least on a lateral or vertical end of an image-pickup tube, thereby producing a signal with a black level corresponding to the optical black mask at the end of horizontal or vertical scanning. The clamp circuit sample-holds the black level and comparing a reference voltage with the level thus held, clamps the optical black level to the reference voltage by such a method as the feedback of the comparison output. This clamp circuit is provided separately for the processing circuit of each of the R, G and B channels of the three primary colors, so that the optical black level of each signal is clamped to the reference voltage.
In the video camera, the black balance indicates the fact that the black levels of the R, G and B channels are arranged in a manner to reduce the color difference signals (R-Y) and (B-Y) of the color encoder input to zero so that the extraneous coloring of the black portion of an object may be avoided or the extraneous coloring of the corresponding portion of the reproduced image may be eliminated when an image of a colorless object is picked up at a place extremely low in illumination. The accuracy of arranging the black levels is required to be as high as about 1% of the reference signal level, and in order to meet this requirement, variable resistors for adjusting the clamp levels of R, G and B channels respectively must be carefully operated. In addition to this problem, it is also necessary to pay adequate consideration and cost to the circuit design and the quality of the parts in order to prevent fluctuations which otherwise might be caused by temperature changes or vibrations of a camera after adjustment. Another disadvantage is that since the Y signal is generally produced by the combination of R, G and B signals, the black levels of R and B signals are correlated with that of the Y signal and therefore the black level of the Y signal is affected by the black level adjustment of the R or B signal, with the result that the adjustment of black level is required to be repeated for the three channels to attain convergence.